feat(lesson-preparation): add readonly view, anchor node selector, and type guards

- Add lesson-plan-readonly-view for viewing published plans

- Add anchor-node-selector and textbook-segments for canvas anchor positioning

- Add i18n-errors and type-guards lib utilities

- Add lesson-plan-provider-setup for provider initialization

- Update actions, data-access (knowledge, versions, main), publish-service

- Update blocks (blackboard, exercise, homework, import, key-point, objective, reflection)

- Update editor, node-editor, node-edit-panel, pickers, and providers

src/modules/lesson-preparation/components/blocks/exercise-block.tsx

@@ -12,8 +12,8 @@ import { Plus, Trash2 } from "lucide-react";
 import type {
   ExerciseBlockData,
   ExerciseItem,
-  ExercisePurpose,
 } from "../../types";
+import { isExercisePurpose } from "../../lib/type-guards";
 
 interface Props {
   blockId: string;
@@ -59,9 +59,12 @@ export function ExerciseBlock({ blockId, data, classes, textbookId, chapterId }:
         <select
           id={`exercise-purpose-${blockId}`}
           value={data.purpose}
-          onChange={(e) =>
-            update({ purpose: e.target.value as ExercisePurpose })
-          }
+          onChange={(e) => {
+            const value = e.target.value;
+            if (isExercisePurpose(value)) {
+              update({ purpose: value });
+            }
+          }}
           className="border rounded px-2 py-1 text-sm"
         >
           <option value="class_practice">{t("exercise.purpose.class_practice")}</option>